Re: [PATCH sched_ext/for-7.3-fixes] sched_ext: Don't report a forced-idle CPU as open for IMMED tasks

[Andrea Righi <arighi@nvidia.com>]

Hi Tejun,

On Sat, Oct 17, 2026 at 10:36:19AM -1000, Tejun Heo wrote:
> Hello, Andrea.
> 
> On Sat, Oct 17, 2026 at 02:47:49PM +0200, Andrea Righi wrote:
> > +static bool higher_class_runnable(struct rq *rq)
> > +{
> > +	if (sched_stop_runnable(rq))
> > +		return true;
> > +
> > +	/* The ext server selects SCX tasks ahead of RT and fair tasks. */
> > +	if (sched_dl_runnable(rq))
> > +		return rb_first_cached(&rq->dl.root) != &rq->ext_server.rb_node;
> > +
> > +	return sched_rt_runnable(rq) || sched_fair_runnable(rq);
> > +}
> 
> This duplicates the core pick order in sched_ext, the dl_server ordering
> included, which is brittle. It also misses the case where the IMMED
> task's own cookie forces the CPU idle, e.g. a tagged IMMED task while
> the sibling runs an untagged RT task, as nothing above ext is runnable
> on the CPU itself.

Ah, right, a tagged IMMED task inserted on a CPU whose sibling runs an
untagged SCHED_FIFO task stays parked on the local DSQ with this patch
applied, which is wrong.

> 
> Instead of predicting the pick, how about reacting to it? A CPU picking
> idle with IMMED tasks on its local DSQ means that they didn't run
> immediately, whatever the reason. scx_update_idle() is already called on
> every idle pick, from set_next_task_idle() on a task to idle switch and
> from pick_task_idle() on an idle to idle re-pick, so maybe something
> like the following in __scx_update_idle()?
> 
> 	if (idle && unlikely(rq->scx.nr_immed))
> 		scx_schedule_reenq_local(rq, SCX_REENQ_...);
> 
> Outside forced idle, nr_immed should be zero there as pick_task_scx()
> would have taken the local DSQ head. The reenq would need a flag telling
> reenq_local() to skip rq_is_open(), as next_class is idle and the rq
> would look open, keeping the IMMED task at the head.

Yeah, makes sense and it looks like a much more robust approach. I'll
send a new version that drops higher_class_runnable() and does this,
with an internal SCX_REENQ_IDLE_PICK (?) flag to make reenq_local() skip
rq_is_open().

Thanks for taking a look at this!

-Andrea